⬆️ Bump sphinx-exercise from 1.0.1 to 1.2.1

Bumps [sphinx-exercise](https://github.com/executablebooks/sphinx-exercise) from 1.0.1 to 1.2.1.
- [Release notes](https://github.com/executablebooks/sphinx-exercise/releases)
- [Changelog](https://github.com/executablebooks/sphinx-exercise/blob/main/CHANGELOG.md)
- [Commits](executablebooks/sphinx-exercise@v1.0.1...v1.2.1)

---
updated-dependencies:
- dependency-name: sphinx-exercise
  dependency-version: 1.2.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

environment.yml

@@ -10,7 +10,7 @@ dependencies:
     - quantecon-book-theme==0.15.1
     - sphinx-tojupyter==0.3.1
     - sphinxext-rediraffe==0.2.7
-    - sphinx-exercise==1.0.1
+    - sphinx-exercise==1.2.1
     - sphinx-proof==0.2.1
     - sphinxcontrib-youtube==1.4.1
     - sphinx-togglebutton==0.3.2